Molten Salt Nuclear reactors have been proposed in several different forms but two main areas differentiate their use. First is how the fissile and fertile materials are carried. Second is whether extra bulk moderator is employed (graphite is typically specified). The first factor sees three potential designs, which are described below.
Single Fluid reactor design: One single salt that contains both fertile (e.g., thorium and/or U238) and fissile material (e.g., U233 and/or Pu239, U235 etc). The benefit of this mode of operation is that typically, the core design is quite simple. The drawbacks include: (1) difficult fission product removal chemistry (as thorium is chemically virtually identical to rare earth fission products) and (2) possibility of a large leakage of neutrons which both lowers the potential breeding ratio and may cause neutron induced damage on the reactor vessel. Examples of single fluid reactors include the circa 1970 Molten Salt Breeder Reactor (MSBR) of Oak Ridge National Laboratories (ORNL) and MOSART of Russia.
Two Fluid reactor design: There are separate carrier salts for the fertile (typically thorium) and fissile material (typically U233). The two main benefits are simpler fission product removal chemistry and greatly reduced leakage of neutrons since they are absorbed in the surrounding fertile blanket. The main drawbacks are: (1) a potentially more complex core, (2) the need for a barrier material between the two salts that can retain strength in a strong neutron flux, and (3) somewhat decreased proliferation resistance understood by those trained in the field since a “blanket” is employed. As an example, a two fluid reactor design was studied by ORNL from 1960 to 1968.
1 and ½ Fluid reactor design (one and a half fluid reactor design): A hybrid design in which a central fuel salt containing both fertile and fissile material is surrounded by a fertile only blanket salt. This has the advantage of decreased leakage of neutrons but fission product removal remains difficult and there is still a barrier material needed between the central region and the blanket region, albeit potentially in a weaker neutron flux than the Two Fluid design. There is also the blanket salt proliferation issue. Examples include ORNL 1954 to 1960, and the French TMSR/MSFR 2005 to present.
In the prior art, the use of a bulk moderator throughout the core (neutron moderator material formed throughout the volume of the nuclear core) can affect reactor design in many ways. Graphite has been by far the most commonly proposed moderator in the core but clad beryllium and/or heavy water has also been investigated. The main effect of having a moderator is a softening of the neutron spectrum, which can allow operation with far less fissile material. A second very important ability enabled by the use of bulk moderator is that it limits neutron leakage by a method referred to as an under moderated outer zone, which is described below.
A Single Fluid design has the drawback that significant numbers of neutrons can be lost to leakage and these same neutrons can damage the outer vessel (typically a nickel alloy such as Hastelloy N). Adding reflector material between the core and vessel wall (i.e., adding a graphite lining to the vessel wall) has only a limited effect as would be understood by workers in the field.
With graphite or other moderator throughout the core, Oak Ridge National Labs proposed an under moderated outer zone in the mid 1960s. They first calculated the ideal ratio of fuel salt to graphite for an infinite core (i.e., no worry of leakage). This led to a specific neutron spectrum, softened by the graphite, which implies that a particular ratio of fertile (typically thorium) to fissile (typically U233) will make the reactor critical. This salt to graphite ratio (typically about 13% salt in most ORNL work) is employed only for the central core. In a thin outer zone (typically about a meter or less in thickness) they used a much higher ratio of salt to graphite (37% in ORNL work). This results in a harder neutron spectrum in this zone and, as would be understood by workers in the field, leads to a much greater absorption of neutrons in the fertile (thorium) versus production in fissile (U233). For an example, see Nuclear Applications & Technology, Vol. 8, February 1970, page 210, FIG. 1. In reactor physics terms this means the inner core has a K infinity of greater than one (net producers of neutrons) while the outer zone has K infinity much less than one (net absorber). The overall combination is a K effective of just over 1.0 as required to maintain criticality.
Three cases relating to an unreflected core, a reflected core and a core with an under moderated outer zone core are shown in FIGS. 1 and 2, which are meant only to show differences in neutron flux profiles for different types of single fluid reactors. In FIG. 1, plot 1 shows the neutron flux for a single fluid molten salt nuclear core being free of any neutron reflector at the periphery of the nuclear core vessel (i.e., in the absence of reflector 4), and plot 2 shows the neutron flux for a nuclear core having a 40 cm-thick neutron reflector 4 at the periphery of the nuclear core vessel. Also shown at FIG. 1 is a wall 3 of the reflector 4. FIG. 2 shows a neutron flux plot 5 (neutron flux profile) for a single fluid molten salt reactor core without a reflector but with an under moderated outer zone 6. As shown in FIG. 2, the neutron flux at the outer periphery (˜200 cm) is greatly reduced in comparison to the unreflected “bare core” plot 1 of FIG. 1.
There are significant drawbacks to using bulk graphite or other moderators (clad beryllium, heavy water). For example, graphite is known to have a limited lifetime in the core which has forced designers to either propose very low power density and thus very large cores or to plan for periodic graphite replacement which is a difficult challenge. As well, the overall safety of Molten Salt Reactors is outstanding but the potential fire hazard of graphite cannot be ignored. Finally graphite use represents a significant disposal. With clad beryllium used throughout the core, the losses of neutrons to the cladding are excessive.
Thus it has long been a desire to be able to design a practical Single Fluid reactor that does not employ bulk moderators such as graphite. However, without an under moderated outer zone, the issue of neutron leakage and damage to the outer vessel have always curtailed these efforts. As well, the less moderated neutron spectrum means a shorter prompt neutron lifetime which has negative implications on reactor control as would be known by those trained in the field. As an example, in the MOSART design of Russia which is a Single Fluid transuranic waste burner, they felt the need to propose two thick layers, a layer of graphite facing the salt to slow neutrons down and reflect some neutrons and then of steel blocks to absorb the unreflected neutrons. This 20 tonne liner of graphite would still require periodic replacement which limits the design's utility. Finally, as would be known by those trained in the art, a graphite reflector can in many cases actually increase the overall leakage of neutrons due to a fission power peaking from more thermalized neutrons re-entering the core salt from the graphite reflector.
Therefore, improvements in molten salt nuclear reactors are desirable.